Abstract: To understand the coupling effect of water and nitrogen on the leaf nutrient contents of Populus tomentosa and to search for an optimal combination of water and nitrogen for P. tomentosa, a field experiment with randomized block design (3 levels of irrigation water and 4 levels of fertilizer nitrogen) was conducted in a two-year-old P. tomentosa clone BT17 plantation in the nursery of Weixian County, Hebei Province of North China from 2007 to 2009. The N, P and K contents in BT17 leaves differed significantly among seasons, with the order of spring > summer > autumn, and had  significant differences among the 12 treatments, being the highest in the treatment 75% of field capacity and 160 g per plant of N application rate. The coupling of water and nitrogen had positive effects on the leaf N, P, and K contents, and thus, more attention should be paid to this coupling effect in the management of P. tomentosa. The leaf nutrient contents had different responses to water and nitrogen. The leaf N and K contents were more affected by soil water content, while the leaf P content was more affected by N application rate.